The fluid catalytic cracking (FCC) unit has been a main processing unit in oil refineries since the 1940's, when the first FCC process was commercialized. Since that time there have been many mechanical and technical improvements in the basic processing system and many processing variations based on fluidization of solid particles have been introduced and commercialized. An inherent problem in all fluidized systems is the control of the particulate effluent, which involves separating the fluidizing media (process gas) and the entrained solid particles.
The original FCC units used small cyclones in the reactor system to separate reaction gases from solids (catalyst) entrained with the gas product from the reactor and returned the catalyst back to the process reactor bed. Because of design considerations, the FCC regenerator recycled solids (catalyst) back to the regenerator from an electrostatic precipitator, which was a principal separation means employed for separating entrained particulates from the regenerator flue gas before venting the flue gas to the atmosphere. As metallurgy and technology improved, the FCC type systems standardized on cyclones as the primary means of separating the circulating solids entrained with the gases from both the reactor and regenerator. Since the late 1940's, the industry standard for the regenerator has been to have two stages of cyclones separating and returning the catalyst to the circulating inventory. The early reactors were designed with only single stage cyclones in the reactor system. Newer FCC reactors have been designed with both single and two stage cyclone systems.
The main problem in design of the cyclone systems for a FCC type system containing a reactor and regenerator is balancing the cyclone efficiencies. That is, the reactor cyclone(s) efficiency, as measured by the size of the particles removed, must be the same or slightly better in the reactor than in the regenerator, as the catalyst losses from the reactor are more difficult to handle than those from the regenerator. In the late 1960's and early 1970's, as more emphasis was placed on environmental considerations and the reduction of particulate emissions from FCC regenerators, the efficiency of regenerator cyclones was improved by the reduction of the gas outlet tube size. However, this improvement in regenerator cyclone efficiency had an adverse effect on the losses from the reactor side and increased the amount of catalyst in the FCC slurry oil. This then resulted in another type of environmental problem; the disposal of increased oily catalyst waste from slurry product tanks. In this same time period there was increased emphasis on increasing FCC plant capacities, so the trend was to go to larger and larger cyclones, which directionally decreases cyclone efficiency and results in higher losses per cubic foot of gas treated.
During the late 1960's and early 1970's, zeolytic catalysts were introduced. This increased the pressure to lower the reactor catalyst losses. With the introduction of zeolytic catalyst, the use of recycle to obtain conversion was not necessary. The use of slurry recycle, which was the method used to recycle reactor catalyst losses back to the reactor system from the slurry settler, was actually discouraged as it resulted in decreased catalytic selectivity and a poorer FCC reactor yield structure.
The pressure, which started in the early 1960's, to increase the capacity of the existing FCC units to their maximum capacity resulted in reaching the mechanical limits on the reactor size. That is, the existing reactor size was not big enough to accommodate any more cyclones or two stages of cyclones. Also, because of the erosive nature of service and, in the case of reactor cyclones, the potential for coking, the industry was not prepared at this time to accept or design cyclones as pressure vessels and to locate the cyclones outside of the reactor or regenerator. Because of these limits, the first riser cracking system was put on stream as a method of reducing the particulate loading of the reactor cyclone system. Since that first riser reactor installation in the early 1970's, there has been a proliferation of ideas for better catalyst and gas separation from the top of the riser. An example of this is the vented riser described in Meyers, et al., U.S. Pat. Nos. 4,066,533 and 4,070,159.
Another critical consideration in the design of both the reactor and regenerator is to design the cyclone systems to maintain a certain particle size in order to maintain circulation and fluidization. As an example, FCC systems employing "U-bends" must operate with a lower Average Particle Size (APS), more in the less than 40 micron APS, than FCC systems that employ shorter and straighter standpipes. For this reason, in particulate fluidization systems the cyclones must have a minimum efficiency. Also, the fresh catalyst supplied must be in a certain particulate size range, as well as meet certain diffusion (activity) and attrition criteria to function in the unit. Thus, the solution to reducing particulate emissions is not as easy as merely using a larger particle.
Recently there have been several notable developments in FCC cyclone technology. One FCC technology is employing external cyclones, but still uses bed rather than transport type two stage regeneration and riser type cracking in the reactor. This type of design has about the same capital requirements as locating the cyclones internally in the reactor and regenerator vessels. When using external cyclones, the FCC units are much more difficult to design because each individual cyclone that was once located inside the reactor and regenerator now becomes a pressure vessel, and while the cyclone inlets and gas outlets can be manifolded, each of the cyclone diplegs must be returned to the vessel. This results in a multitude of cyclone diplegs than need to return the catalyst from each of the first and second stage cyclones back to the vessel at the same elevation in the vessel. These returns and their resulting reinforcing pads usually sets the size (diameter) of the vessel to accommodate the openings, and therefore, there is little or no savings in this type of design.
Another cyclone system of note is the Euripos third stage cyclone described in U.S. Pat. No. 4,348,215. The reasons this system is used as a third stage system and not as a primary or secondary system to separate the catalyst from the effluent gas and return the catalyst to the circulating inventory are two-fold. First there is a definite limit on the particulate concentration (loading) the system can handle without flooding, and bed type regenerators, which is today's prevalent technology, are prone to high particulate concentrations. Secondly the efficiency of the system as described is too high. If one could overcome the particulate loading problem, the second problem of maintaining the desired balance in the reactor and regenerator cyclone efficiencies is a major concern. The concern over potential coking problems in the reactor has eliminated this type of system from consideration for the reactor.
The use of this high efficiency Euripos type system on the regenerator and not the reactor, which would be the same as recycling the tertiary or precipitator fines back to the process unit in today's state of the art units, would result in increased catalyst losses from the reactor because of the imbalance of the cyclone efficiencies. Also, it is often believed that the increased barrel velocities, which are the source of the increased efficiencies of the smaller cyclones, will result in increased attrition and higher catalyst losses if used On the circulating inventory.
Besides decreasing the gas outlet tubes for increased efficiency, the industry has been using the same cyclone technology in both the reactor and regenerator since the 1950's, and coping in different ways with the reactor catalyst carryover into the slurry product.
Thus, a general objective to the present invention is to overcome the aforementioned and other related problems in prior art systems for separating particulates from process gas streams.
A primary object of the present invention is a method and apparatus for controlling particulate emissions in a system employing a gaseous fluidizing media to transport particulate solids.
Another object of the invention is an improved solids-gas separation system which achieves an increased efficiency of particulate removal from a fluidized solids process stream.
Still, another object of the invention is a solids-gas separation system which enables decreased capital requirements when employed in a process unit using a gaseous fluidizing medium to transport particulate solids.
Yet, another object of the invention is an FCC or 3D type (as hereinafter described) processing system which utilizes only one stage cyclones, as compared to current state of the art processing systems utilizing two stages of cyclones, to achieve a very low level of particulate solids in regenerator flue gases exhausted to the atmosphere and/or in reactor effluent vapors.
Other objects and advantages of the present invention will become apparent from the following description thereof and from the practice of the invention.